1. Technical Field
The present invention relates in general to centrifugal pumps and, in particular, to an improved system, method, and apparatus for a two-phase homogenizing stage for a centrifugal pump assembly.
2. Description of the Related Art
Centrifugal pumps are primarily designed to handle liquids. However, in the presence of liquids having high percentages of free gas, centrifugal pumps will suffer from pressure degradation and gas locking. Moreover, extreme differences in the densities of the liquids and gases cause the gases to gather in the low pressure areas of the pump adjacent the impeller eye. This results in gas accumulation in the impeller and blocks the flow path for the liquids. Furthermore, free gas in the impeller stages displaces liquid and restricts the volumetric efficiency of the pump. As a result, the accumulation of free gas results in lower volumetric lift per pump stage, and a decline in expected production.
The theoretical best performance of a pump in a two-phase fluid is represented by what is known as the “homogeneous curve.” The homogeneous performance of a pump is based on the hypothesis that if the size of gas bubbles is reduced to the point that the fluid drag forces completely dominate the buoyant forces, the two-phase fluid would behave as if it was a single-phase fluid whose only effect on performance would be the increased volume and the reduced bulk density. The two-phase performance of a pump can approach the homogeneous curve if the fluids can be homogenized and the homogeneity is maintained throughout the pump.
As shown in FIG. 10, pump performance curves are based on the assumption that the gas entrained in the liquid affects only two variables: (1) the volume of the total mixture, and (2) the density of the total mixture. Graphed against the liquid-only volume and the stage pressure, a curve paralleling the “all liquid” performance plot 101, but intersecting the horizontal flow axis at a value of “max liquid flow×1−Free Gas Fraction (or Gas Void Fraction),” and intersecting the vertical pressure axis at “max pressure×1−Free Gas Fraction” is shown. Other plots 103, 105 are shown for values of 10% and 25% free gas, respectively. The actual performance 107 is more severely affected by the gas as shown by the “Realistic curve at 25% free gas.” If the gas cannot be mixed and carried through the pump, it tends to centrifugally separate, gather in the eye of the impeller and can block the liquid flow entirely. The term “shut in” or “shut off” refers to the flow control valve being shut. It represents the performance curve intersection at the vertical (zero flow) axis. Thus, a system, method and apparatus for improving the performance of centrifugal pump assemblies in two-phase fluid production would be desirable.